Speed-indicator for electric motors.



PATENTED JULY 21, 17903.

E. J. BERG. SPEED INDICATOR FOR ELECTRIC MOTORS.

APPLICATION FILED AUG. 24, 1898.

N0 MODEL.

Elfins J. Berg,

v tit 1 I, UNITED STATES PATENT Patented Jul 21, 1905.

()FFICE.

ERNST J. BERG, OF. SOHENECTADY, NEW YORK, ASSIGNOR TO THE GENERAL ELECTRIC OOMPANY,-A CORPORATION OF NEW YORK.

SPEED-INDICATOR FOR ELECTRIC MOTORS.

SPECIFICATION forming part of Letters Patent No. 734,294, dated Jul :21, 1903. Application filed August 24,1898. Serial No. 689l3'73. (N0 model.) I

To all whom it may concern:

Be it known that I, ERNST J. BERG, a subject of the King of Sweden and Norway, residing at Schenectady, in the county of Schenectady and State of New York, have invented certain new and useful Improvements in Speed-Indicators for Electric Motors, of which the following is a specification.

In the operation of induction-motors it is well known that the speed drops as the load increases. This drop in speed may be considerable in some instances without being soriously detrimental. In some cases, however, a drop in speed as great as two or three per cent. is not permissible, and this isparticularly true in cases where induction-motors are used for running cotton or other textile machiuery,where even a slight speed variation mayhave a very destructive influence. In order to fulfil the requirement of close speed regulation thus called for, it is necessary to make the motors very large, in which case they become very expensive in proportion to the amount of power developed. Instead of providing large motors with close regulation the same object maybe accomplishedby using smaller or less expensive motors and preventing too great a drop in speed by correspondingly speeding up the generators in the power-station. To do this, it is necessary to have in the power-station a device for indicating either the. speed. or drop in speed of the motor in the receiving-station. A device of this character is described and claimed in this application.

The uses to which the device may be put are many and various, so that for purposes of illustration I have described only onerelation in which my invention is of value, although it will be evident to those skilled in the art that the device is in no way limited in its application thereto, but may be used for many other purposes.

In the drawings, Figure 1 is a diagrammatic representation of my invention, and Fig. 2 is a view of one form of apparatus which I employ.

In carrying out my invention for indicating at a distance the speed of an inductionmotor I make use of the fact that currents are developed in the induced member of the slip.

motor of a frequency corresponding to the slip, or, in other words, to the difference between the actual speed ofthemotor and that which it would have-at absolute synchronism. By tapping the windings of the induced mernber at suitable intervals currents may be taken ofi of a frequency corresponding to the If these currents be conducted to any desired point and the frequency of the currents there measured, a result is obtained indicative of the speed of the induction-motor.

In the drawings, Irepresents the inductionmotor, the speed of which is to be measured. The induced member 1 of the motor is tapped at points separatedfrom each other by an angle equal to one-third of the polar pitch.

Three-phase currents are taken off through collector-rings 2 and brushes 3. Owing to the low frequency at which these currents are generated their voltage may. be too low to economically run the frequency-indicator F.

To remedy this objection, a step-up trans former T of any suitable character may be employed. The secondary mains 4 lead to the frequency-indicator F, which, as beforestated, may be located at any desired point. The frequency-indicator may assume many forms without departing from the spirit of my invention; but the form which I prefer to employ and which I haveillustrated in the drawings consists of a motor of the rotaryfield type, in which one member 5 is fed with p'olyphase current of the same frequency as that fed to the motor whose speed is to be measured, while the other member 6 is fed with current from the step-up transformer T, the latter current, as before stated, having a frequency dependent upon the slip of the motor. It is within my invention, however, to construct the frequency-indicator in the form of a synchronous motor, in which the field is fed by direct current from any suitable source. In this case the speed of the frequency-indicator corresponds to the slip of the induction-motor.

In the normal operation of the preferred form of frequency-indicator F, in which both members are supplied with energy from the same mains, the polar lines produced in its two relatively rotatable members will coinroo cide' and the speed of the revolving member will be such as to accomplish the result. Thus if the polar lines have the same direction of rotation in space then the revolving member will have a speed less than that of synchronism by an amount equal to the slip of the motor I. In other words, the speed of the frequency-indicator F is the same as that of the motor whose speed is to be measured, so that all that is necessary in order to determine the speed of the motor I is to measure the speed of rotation of the revolving member of the frequency-indicator F. This may be done by a tachometer 7 of any suitable character or by any other known means for measuring the rate of rotation of a shaft. The reading of the tachometer gives the speed of the motor I. If the number of poles on the frequency-indicator F and the motor I differ, this reading will require to be increased or decreased accordingly; but in any case it will be proportional to the speed to be measured. If the polar lines of the two relatively rotatable elements of the frequency-indicator F revolve in opposite directions, then it is evident that the revolving member will rotate at a rate equal to the sum of the rates of rotation of the polar linesthat is, at a speed equal to the speed of synchronism plus that corresponding to the slip of the motor I. The reading of the tachometer minus a constant corresponding to the speed of synchronism gives the slip of the motorI. Either method of speed measurement may be employed, as convenience may dictate. If it be desired to measure the variation in speed of the machine from the speed corresponding to synchronism, it is necessary to obtain two readings from the frequency-indicator, one with the two polar lines revolving in the same direction, the other with the two polar lines revolving in the opposite direction. One reading represents the speed of synchronism minus the slip of the machine whose speed is to be measured, while the other reading represents the speed of synchronism plus the slip. One-half of the difference between these readings obviously represents the variation in speed from synchronism of the machine Whose speed is being measured. The mean of the two readings gives, of course, the speed of synchronism. In order to reverse the direction of rotation of the polar lines in either of the two members of the frequencyindicator in order to obtain these two readings, it is only necessary to transpose the connections of two of the leads supplying current to that member.

Although I have described two methods of measuring the speed of an induction-motor, one of which gives directly either the exact speed of the rotating member or a multiple thereof, while the other gives a result corresponding to the speed of synchronism plus a quantity equal to the slip or drop in speed of the motor, I prefer, ordinarily, to employ that method of speed measurement which gives directly the speed of the motor rather than resort to a method which presupposes a knowledge of the speed of synchronism, since this speed cannot be definitelyknown unless specially measured. It is obvious, nevertheless, that the methods of speed measurement described by me other than that which I prefer are equally within my invention.

It will be evident to those skilled in the art that my invention is not limited in its application to the measurement of the speed of induction motors, but may be employed to measure the speed of any device in which alternating currents are developed of a frequency dependent upon the speed. Thus, for example, the armature of a direct-current motor may be tapped at suitable intervals and the resulting alternating current fed to one member of a synchronous motor, the field of which is excited by direct current. The speed of the synchronous motor will correspond to the speed of the armature of the direct-current machine. In the same manner a synchronous motor might, as before stated, he used in place of that form of frequencyindicatorillustrated in tlie drawings. Under such circumstances the synchronous motor would indicate simply the slip of the inductiou-motor rather than its actual speed.

Many other m odifications will suggest themselves without departing from the spirit of my invention.

What I claim as new, and desire to secure by Letters Patent of the United States, is

l. The combination of an alternating-current dynamo-electric machine each member of which is provided with a multiphase winding, a source of current of a given frequency connected to one winding, a source of current of a different frequency connected to the other winding, and means for measuring the relative motion of said members.

2. The combination of an induction-motor, conductors connected at suitable intervals to the winding on the induced member of the motor, and a rotary-field speed-indicator in electrical connection with said conductors.

3. The combination of an alternating-current dynamo -electric machine having two relatively movable members, a source of current of given frequency connected to one member, a source of current of difierent frequency connected to the other, and means for measuring the relative motion of said members.

4:- The combination of polyphase mains, an induction-motor having its inducing member connected thereto, conductors connected in polyphase relation to the induced member of said motor and leading therefrom to one of the two relatively revolving members of a rotary-field speed-indicator, and connections from the other member of said indicator to said polyphase mains.

5. The combination of a rotary-field induction-1notor, conductors connected in polyphase relation to the induced member of said motor and leading therefrom to one of two relatively rotating members of a frequencyfor supplying current of one frequency to one measuring device, means for developing in member of the machine, means for supplying the other member of said device a rotary field current of differing frequency to the other of the same frequency as that of the current member, and means for indicating the revo- 15 5 supplied to the induction-motor, and means 'lutions of said machine.

for measuring the speed of the rotating mem- I In witness whereof I have hereunto set my ber of said frequency-measuring device. hand this 22d day of August, 1898.

6. A device for measuring the difference in ERNST J. BERG. frequency between two alternating currents Witnesses: 10 which consists of a dynamo-electric machine B. B. HULL,

having relatively revolving members, means 1 M. N. EMERSON. 

